1. Field of the Invention
The present invention relates to high-availability computer systems, and more particularly to computer systems with failover capabilities.
2. Background of the Related Art
The availability of a computer system generally refers to the fraction of time during which a computer system remains operational for its intended use. A computer system may undergo periods of scheduled or unscheduled downtime, during which the computer system is unavailable. Scheduled downtime may result, for example, from periodic maintenance or system changes that require shutting down the system. Unscheduled downtime events typically arise from some unplanned physical event, such as a hardware or software failure or environmental anomaly. Examples of unscheduled downtime events include power failures, hardware failures, a shutdown due to temperatures in excess of a threshold, severed network connections, security breaches, or various application, middleware, and operating system failures.
High availability (HA) refers generally to the ability of a system to remain available for its intended use during a given measurement period. A business providing commercial access to datacenter resources may promise a certain degree of operational continuity during a contractual measurement period. For example, a datacenter may promise an availability of 99%, which corresponds to no more than 1.68 hours of downtime per week. Some vendors exclude planned downtime from the measurement period, to increase the rated availability of a system, in which case the availability is determined solely by the amount of unplanned downtime.
One way to avoid unplanned downtime is to avoid system shutdowns through the use of a failover system. A failover refers to automatically switching over to a redundant or standby computer server, system, or network upon the failure or abnormal termination of the previously active server, system, or network. A variety of methods are known in the art for transferring workload from one server to a redundant server in the event of a failure. Redundant servers and other failover equipment consume additional power, even though by its nature, failover equipment may rarely be used. The cost of this additional power factors into the total cost of ownership of a computer system.